In the field of optical position encoders, for example incremental optical position encoders, it is known to include markers on or with an optical scale that is used to derive fine-grained position information. The markers may include, for example, an index marker that establishes a “home” or index position of the encoder from which all position measurements or indications are referenced. So-called “limit” markers have also been used, especially in linear position encoders, to provide an indication when the encoder has reached a limit of its travel. Other types of markers have also been employed.
There are alternative techniques for incorporating a marker with an optical scale. In a typical approach, a marker is simply a special feature of the scale itself, and is formed at the same time and by the same process by which the regular scale features (e.g., regularly spaced lines) are formed. The marker may be a set of separate elements apart from the scale marks, or they may be specially formed scale marks, such as a set of relatively longer or shorter marks or some other modification of the basic mark structure. In all these cases, the marker is an integral part of the scale itself, and thus the scale as manufactured has a predetermined arrangement or marks. This makes such scales to some extent “custom”—they are designed with particular applications in mind that utilize certain specific arrangements of markers. If a scale vendor is to sell products to many customers having a variety of applications, it is necessary for the vendor to manufacture and stock a corresponding variety of scale types, which can contribute to increased cost for engineering, manufacturing, and customer support.
Alternatively, it has been known to manufacture scales with multiple markers formed at the same time as the scale at predetermined locations, and then for a customer/user to choose which of the markers matches his/her application and then selectively remove those that are not needed. The unused markers are either physically covered or removed. It is noted that this method is not fully-customizable, as a customer's choices for marker position are limited to those predetermined locations selected by the scale manufacturer.
It is also known to create a scale assembly by incorporating separate markers onto scales that are unmarked as manufactured (i.e., scales that have no markers apart from the regularly spaced scale marks). For example, in one arrangement a magnetic element is placed immediately adjacent to an edge of a linear scale of a linear optical encoder. The element can be detected by a magnetic detector that is co-located with the optical detector that reads the regular optical scale marks. Other approaches using markers entirely separate from the optical scale are also known.